It is common for a cable to include elements (e.g., subunits) that can be extracted from the cable. Typically, these elements extend side-by-side within the cable (e.g., substantially in parallel within a central cavity defined by the cable's inside wall). Generally, the elements are freely positioned within the cable so that an operator can extract them easily from the cable. In other words, the elements can freely move within the cable's central cavity.
Such cable designs are commonly used for optical-fiber telecommunications networks. By way of example, an optical-fiber cable may include elements (e.g., optical modules or buffer tubes) that are designed to be extracted from the cable in order to serve different buildings. For this purpose, an operator may create an opening in the cable to extract one or more modules that are then taken to a building that is to be served. Other modules can remain within the cable to be extracted (e.g., available to serve another building).
Such optical-fiber cables typically have an outer sheath that defines a central cavity in which optical-fiber modules may extend. Each module typically includes one or more optical fibers, usually between one and twelve optical fibers, wrapped together within the center of the module. The outer sheath is typically made of a polymer (e.g., polyethylene). The outer sheath may also include reinforcements (e.g., strength members) extending longitudinally along the axis of the cable. Such reinforcements may serve to stiffen the structure to prevent the cable from deforming excessively under the effects of temperature or an external force (e.g., from traction or compression).
When an operator in the field seeks to extract a length of an element (e.g., a module) from the cable, the length of an element that can be extracted is limited by several factors. By way of example, the packing ratio of the elements in the cable may limit the length of a module that can be extracted. The packing ratio is defined as the ratio of the sum of the cross-sectional areas of all of the elements divided by the cross-sectional area of the cavity. Other factors that can affect the length of an element that can be extracted include the coefficient of friction between two elements and the coefficient of friction between an element and the wall of the cable cavity.
By way of example, commonly owned European Patent Application Publication No. EP 1921478 A1, which is hereby incorporated by reference in its entirety, discloses a cable including modules and having a maximum packing ratio of 0.7, with the ratio preferably lying in the range of 0.3 to 0.35. The cable includes a solid lubricant (e.g., talc) placed within the cavity to reduce the coefficient of friction between two modules and the coefficient of friction between a module and the wall of the cavity. In the absence of a solid lubricant, the coefficient of friction between the modules (e.g., made of polyester based on thermoplastic elastomers or plasticized PVC) and the inner wall of the cable was high.
Nevertheless, the cable described in European Patent Application Publication No. EP 1921478 A1 has several drawbacks. For example, it requires a large amount of solid lubricant in the cavity to ensure that lubricant is present at all places between two modules. Furthermore, although the length of module that can be extracted may reach 30 meters (m), which is sufficient to enable a cable to be installed in an external duct such as a sewer, such a length is not sufficient for all kinds of applications.
Proposals have also been made to use materials such as polyester or polyamide for the modules in order to reduce their respective coefficients of friction relative to the cable. Thus, if a solid lubricant is provided in insufficient quantities or distributed with insufficient uniformity in the cavity containing the modules, the absence of solid lubricant can be at least partially mitigated so as to retain an acceptable extraction length.
The foregoing notwithstanding, prior cable designs do not allow modules to be extracted from a cable at a sufficient length for all applications (e.g., a branch connection). Indeed, prior solutions do not enable extraction lengths of more than about 30 meters, because the extraction force required becomes too great and often leads to the extracted element being damaged or broken. Accordingly, a need exists for cable designs that better facilitate module extraction.